This invention relates to localized vacuum processing of a workpiece and, more particularly, to apparatus for positioning a workpiece in proximity to a localized vacuum envelope so as to form a noncontacting vacuum seal as the workpiece is translated laterally with respect to the vacuum envelope.
Particle beam systems, such as ion implanters, electron beam lithography systems and ion beam lithography systems, require evacuation of the region along the path between the particle beam source and the workpiece being treated. The workpiece is introduced into the vacuum chamber either directly or through an air lock. Such systems are highly complex and expensive. In addition, the vacuum pumping time causes a reduction in overall processing speed, a factor of utmost importance in a commercial semiconductor processing environment. These problems have been alleviated by the development of localized vacuum processing as described hereinafter.
Certain particle beam processes require movement of the workpiece during processing. In electron beam lithography, microminiature patterns of extremely high accuracy are exposed on a workpiece. To achieve high accuracy, the electron beam deflection field is limited to an area much smaller than the area of the workpiece. Typically, an electron beam deflection field is a few millimeters on a side, while the workpiece, usually a semiconductor wafer or a mask plate, can be several inches in dimension. In order to expose the entire workpiece, precise positioning of the workpiece with respect to the electron beam is required. In the past, the workpiece has been positioned by a system of drive motors external to the vacuum region; and a mechanical linkage has been used to transmit the motion into the vacuum through bellows or rotary vacuum seals. Such systems have been large, complex and relatively expensive.
Apparatus for vacuum processing in a localized region on the surface of a workpiece is disclosed in pending application Ser. No. 435,179, filed Oct. 19, 1982, and assigned to the assignee of the present invention. The vacuum processing apparatus includes an envelope which defines an internal vacuum processing zone. The tip of the envelope is positioned just above the surface of the workpiece and is spaced from the workpiece by a preselected gap. The tip of the envelope, together with the workpiece, forms a noncontacting graded vacuum seal between the internal vacuum processing zone and the ambient environment. The vacuum envelope provides a vacuum zone on the surface of the workpiece which is small in comparison with the size of the workpiece.
In order to treat the entire surface of the workpiece, the workpiece is mounted on a stage which is movable laterally with respect to the tip of the envelope. To achieve rapid processing in electron beam lithography systems, stage speeds of between 1 and 10 cm/sec are typically employed. During this movement, the gap between the tip of the envelope and the workpiece must be dynamically controlled within specified limits. If the gap becomes too large, the vacuum in the vacuum zone is reduced; and processing is interrupted until the required vacuum level can again be achieved. Electron beam columns used in lithography systems must be operated in the mid to high vacuum range. Typically, inadequate space is available to provide high conductance vacuum pumping. In such cases, the gap must be relatively small. When high conductance vacuum pumping can be provided or lower pressure can be tolerated by the process, a larger gap is permitted. Conversely, the gap cannot become so small as to risk contact between the tip of the envelope and the workpiece. A workpiece, such as a semiconductor wafer, is extremely fragile and can be permanently damaged or broken by such contact. While semiconductor wafers are in general flat and planar, they are subject to thickness variations, surface irregularities and process induced warping, all of which can cause gap variations during the lateral movement of the wafer. Furthermore, the stage system which provides the lateral movement of the wafer with respect to the tip of the envelope is subject to certain imperfections and tolerances which can cause the gap to vary as the wafer is moved. These factors make is necessary to dynamically control the gap between the tip of the envelope and the workpiece during processing.
A charged particle beam system providing lateral movement of the workpiece during localized vacuum processing is disclosed in pending application Ser. No. 435,178, filed Oct. 19, 1982, and assigned to the assignee of the present invention. A gap control system for localized vacuum processing is disclosed in U.S. Pat. No. 4,528,451 assigned to the assignee of the present invention.
A critical component of the workpiece positioning system is the z-axis actuator apparatus which mechanically positions the workpiece, with respect to the tip of vacuum envelope in response to a control signal derived from a gap measurement. The actuator must be extremely accurate, since the gap between the workpiece and the vacuum envelope must be maintained within a few micrometers of a prescribed value. In addition, the actuator must be fast acting in order to maintain the required gap during lateral movements at speeds up to 10 cm/sec. Furthermore, the actuator apparatus must move the workpiece along a z-axis without introducing tilt or rotational movement, which would result in distortion and misplacement of the patterns written by the electron beam.
U.S. Pat. No. 4,528,451 discloses the use of piezoelectric z-axis actuators. While the disclosed apparatus provides generally satisfactory performance, piezoelectric actuators require relatively high operating voltages and require means for amplifying their relatively small mechanical movement. When the movement is amplified, the actuator force is correspondingly reduced.
It is a general object of the present invention to provide novel apparatus for positioning a planar workpiece.
It is also an object of the present invention to provide novel apparatus for positioning a workpiece with respect to a localized vacuum envelope.
It is another object of the present invention to provide apparatus for maintaining a prescribed gap between a workpiece and a localized vacuum envelope during lateral translation of the workpiece.
It is yet another object of the present invention to provide apparatus for z-axis positioning of a workpiece relative to a localized vacuum envelope without introducing unwanted lateral or rotational workpiece movement.
It is still another object of the present invention to provide apparatus for hydraulic z-axis positioning of a workpiece, with respect to a localized vacuum envelope.